1. Field of the Invention
The invention relates to golf devices, and more specifically to a device for measuring the acceleration of a golfer""s swing for the purpose of matching a golf club having a desired frequency of oscillation to the golfer""s swing.
2. Description of the Related Art
A golf club is a flexible rod which, when driven toward a ball by the golfer""s hands, bends like a spring under the force applied to it. As the club head advances rapidly toward the ball, the shaft straightens. At some point in the swing, the striking face of the club head strikes the ball.
Striking faces are designed to strike a ball when the club""s shaft is straight, imparting the desired loft to the ball due to the angle of the striking face at impact. If the ball is struck when the shaft is bent, the club head""s striking face is not properly angled relative to the ball. A misaligned striking face results in an unsatisfactory drive, and therefore it is preferred that a club shaft be straight when the striking face and ball meet.
Additionally, when the shaft is straight, maximum club head velocity relative to the rest of the shaft is obtained. For this reason also a straight shaft is preferred at ball impact.
Some shaft deflection may be caused by centrifugal force when the center of gravity of the golf club head is not aligned with the shaft axis. However, this phenomenon is not considered here as part of the ball striking mechanism due to its unpredictability and unquantifiability. For the purpose of the present invention, the center of gravity is assumed to be aligned along the shaft""s axis.
Because a shaft is bent early in a swing, the shaft must straighten as the swing proceeds. In order for a bent shaft to straighten between the time of maximum acceleration and ball impact, the shaft must be very carefully matched to the characteristics of the golfer""s swing. Such matching requires consideration of the physical properties of the club and the application of force to the club through the golfer""s hands.
A golf club has a rest axis, which is the axis of the shaft when the shaft is straight. The force applied through the golfer""s hands bends the shaft from the rest axis. The force due to acceleration of the club by the golfer results in a bending force applied to the club, thereby bending the shaft from the rest axis.
When the acceleration of the golf club is at its maximum, and therefore the force due to acceleration is at its maximum, the shaft is bent the maximum amount. At maximum acceleration the velocity of the club head relative to the rest axis is zero, because the club has not begun to straighten from its most bent position.
When acceleration begins to decrease from a maximum, the force bending the shaft decreases, and the shaft begins to straighten. At this time, relative velocity begins to increase from zero. As acceleration continues to decrease, velocity continues to increase, with a maximum relative velocity occurring when acceleration of the head with respect to the shaft reaches zero.
A club shaft can only be straight at the time of ball impact if the club has a shaft stiffness and club head mass matched to the golfer""s swing. The stiffness and mass properties determine how fast the club returns to straight from bent. Only if these properties are matched to the manner in which the golfer applies force to the club will the club have a straight shaft at ball impact.
A method of determining the golf club properties necessary to return the golfer""s shaft to straight at ball impact is the subject of U.S. Pat. No. 5,351,952 to Hackman, which is incorporated by reference. In this patent, a method is discussed for calculating a natural frequency of oscillation of the golf club that matches the swing time of the golfer""s swing. If a club is built to the frequency that is determined mathematically to match the golfer""s swing, then it will return to straight by the time of ball impact.
Although it is possible to measure the characteristics needed to determine the frequency a golf club must have to match a particular golfer""s swing, it is somewhat difficult. In conventional measurement devices, an accelerometer is fastened to the club head of a golf club, and is connected by wires (or wireless transceiver) to a computer.
This apparatus is functional, but its use is limited to an artificial environment, such as an indoor booth or driving range. In such an environment, the golfer may not actually swing in a normal manner, despite the efforts of the golfer to reproduce his normal swing.
U.S. Pat. No. 5,478,073 to Hackman, which is also incorporated by reference, discloses a method of calculating the length of that portion of a shaft that must be removed in order to give the shaft the desired final frequency. This process, known as xe2x80x9ctippingxe2x80x9d involves cutting off a small portion of the shaft to increase the shaft""s stiffness, and therefore its frequency of oscillation. The shaft is tipped by the calculated amount and a club head is attached, resulting in a club that matches the golfer""s swing. Such a matching club will straighten by ball impact from bent at maximum acceleration.
The need exists for an apparatus which can be used during normal golf activities, so as to encourage the golfer to swing in the normal playing manner, thereby making for a more accurate match between golf club and golfer.
The invention is a golf swing analyzer attachable to a golf club. The analyzer comprises a housing and a shaft mount attached to the housing for attaching the housing to a golf club shaft. An accelerometer and microprocessor are mounted within the housing. The microprocessor is connected to the accelerometer for processing acceleration measurements made by the accelerometer. A data storage is mounted within the housing and is connected to the microprocessor. The invention also comprises an output port connected to the microprocessor. In one embodiment, the shaft mount further comprises a clamp having a pair of opposed lips for positioning on opposite sides of the golf club shaft and clampingly gripping said shaft between said lips.
In a preferred embodiment, the housing comprises a pair of opposed shells pivotably mounted together along one edge at a hinge. Each shell has a shaft-receiving surface at its opposite ends. The shaft-receiving surfaces of the opposed shells seat against, surround and frictionally grip a golf club shaft when the analyzer is mounted to the shaft in an operable position. One of the opposing shells of the analyzer has C-shaped retaining latches pivotably mounted thereto for pivoting around a portion of each shell""s shaft receiving surface when the analyzer is mounted to a golf club shaft in an operable position.
The invention also contemplates a method of matching a golf club of a preferred frequency to a golfer""s golf swing. The method comprises the golfer swinging a golf club, and measuring the acceleration of the golf club at multiple times during swinging. A maximum acceleration datum is selected, as are a first datum and a second datum. The first datum is measured a predetermined time before the maximum acceleration datum, and the second datum is measured a time after the maximum acceleration datum substantially similar to the predetermined time. A substantially parabolic curve is fit through the first datum, the second datum and the maximum acceleration datum. A measurement time of a new maximum acceleration datum is determined at the peak of the parabolic curve. The method includes determining a swing time between the time of the new maximum acceleration datum and ball impact, and calculating swing frequency by the equation: frequency=15,000÷(swing time).
An alternative method of determining the frequency of the properly matched golfer""s club is contemplated. The method includes two steps: calculation of swing time; and calculation of a swing time using a correction factor, k. In calculating the swing time, a candidate for a release point is selected and a sine wave curve data set is constructed from an equation and the angular acceleration data set. These data sets are subtracted from one another and the differences are summed. If the sum ever reaches a predetermined value, the candidate is rejected. If not, the candidate is accepted and the swing time is calculated with a correction factor that is based upon an equation that is a function of the club""s velocity at the release point.